I didn't see pics of the black rat either, just the nice rosy. In any case, black rats are often in the classifieds. Maybe not every week, but they do show up. Some white patterning on the first 1/3rd of the snakes is not uncommon. Also, there is a "rusty" morph which is a hypo.

A number of morphs in black rats exists actually. There is a large pet store in so cal that had some black rat adults of some different morphs. Really neat snakes. Get big.